As is widely known, a liquid crystal display device is used in large numbers as a visual display device of a computer or the like and is expected to be used more widely also for use in television in future. However, a TN type cell that at present is widely used has serious problems with its display performance when used as television, for example, a narrow view-field angle, an insufficient response speed, deterioration in parallactic contrast, and blurring of moving images.
In recent years, studies on an OCB cell that is to be used instead of such a TN type cell have been made. The OCB cell is characterized by having a wide view-field angle and a high-speed response compared to the TN type cell, so that the OCB cell can be regarded as a liquid crystal cell that is more suitable for displaying natural moving images.
In the following, a conventional method of driving a liquid crystal display device and a liquid crystal display device will be described.
FIG. 14 is a block diagram showing the configuration of a conventional liquid crystal display device.
In FIG. 14, X1, X2, . . . , Xn are gate lines, Y1, Y2, . . . , Ym are source lines, 126 is a thin-film transistor (hereinafter referred to as a TFT) as a switching element, and a drain electrode of each TFT is connected to a pixel electrode inside a pixel 106. Each pixel 106 includes a pixel electrode, an common electrode that is a transparent electrode and a liquid crystal interposed between these two electrodes. The common electrode is driven by a voltage (Vcom) supplied from an common driving part 105. The voltage Vcom supplied to the common electrode has two kinds of voltages, including a first reference voltage Vref1 and a second reference voltage Vref2, and the voltage is supplied while switching between them for each horizontal period.
103 is an IC (hereinafter referred to as a source driver) that outputs a voltage to Y1, Y2, . . . , Ym to be supplied to the pixel 106. 104 is a gate driver for applying a voltage rendering the TFT 126 to be in an ON state or a voltage rendering the TFT 126 to be in an OFF state. The gate driver 104 applies an ON voltage sequentially to the gate lines X1, X2, . . . , Xn in synchronization with the supply of data to the source lines Y1, Y2, . . . , Ym by the source driver 103. A phase of the voltage supplied from the source driver 103 is opposite relative to a phase of the voltage Vcom supplied to the common electrode. This difference in voltage between the voltage Vcom supplied to the common electrode and the voltage applied to each pixel 106 via the source lines Y1, Y2, . . . , Ym is a voltage that is applied to both ends of the liquid crystal inside the pixel 106, and this voltage determines the transmittance of the pixel 106.
In addition, FIG. 15 is a diagram showing waveforms of the voltage Vcom supplied to the common electrode, a source signal Vs serving as an image signal (VI) supplied to the source driver 103, gate signals Vg(n−1), Vg (n), Vg (n+1) applied to (n−1) line, n line, (n+1) line respectively and the timing relationship thereof.
Such a driving method is the same when using the OCB cell as well as when using the TN-type cell. However, the OCB cell needs to be driven in a special manner at an operation stage of starting an image display, which is not required for the TN type cell.
As shown in FIG. 16, an OCB cell has bend configuration (white display) corresponding to a state capable of an image display (FIG. 16B), bend configuration (black display) (FIG. 16C) and splay configuration corresponding to a state incapable of displaying (FIG. 16A). In order to shift the configuration from this splay configuration state to a bend configuration state (hereinafter referred to as a transition), special driving needs to be done such as application of a high voltage for a fixed period of time. However, the driving concerning this transition is not directly related to the present invention, so that it will not be further explained.
However, the problem with this OCB cell was that even if transition to bend configuration is once effected by the aforementioned special driving, when a condition continues to proceed in which a voltage of a predetermined level or higher is not applied for longer than a fixed period of time, the OCB cell cannot maintain the bend configuration and thus returns to the splay configuration (hereinafter, this phenomenon is referred to as a back transition).